Some spellers may only be able to work with certain aspect/element of reality, or a person may only be interested in their ability to work with that particular aspect/element. This is their domain - the range of what they can do with their magic. It could be that they have focused on training / learning for working with spells pertaining to that aspect/element, and become specialized. Or it could be that of the many aspects/elements, they are only successful with one or a few of them. The domains are defined differently in different factions, according to their paradigms; ie one faction may group all elements/effects into a single domain, whereas another may have one domain for fire, another for cold, etc.
For those with limited (or a single) domain, the ‘spell’ in ‘spell~er’ gets replaced with the name of the aspect/element, ie ‘waterdancer’, ‘firestriker’, ‘healthsinger’, etc. As with the ways and styles, people with different domains within a sect will have various titles depending on their sect, their style and their domain.